1. Field of the Invention
The present invention concerns vibration motors.
2. Description of the Prior Art
Vibration motors are also known as ultrasound motors, referring to their preferred frequency, or piezo-active motors, referring to their preferred excitation material.
Several vibration motor structures having a high power to weight ratio combined with the ability to generate much higher mechanical powers than prior art vibration motors have already been proposed. See, for example, French patent applications 95-14.1169 and 97-10.948.
One particularly advantageous application of these motors concerns secondary flight controls in aircraft.
An aim of the invention is to propose improvements to vibration motors and in particular to optimize the efficiency of these motors.
Vibration motors conventionally combine tangential vibration modes, which correspond to deformations in directions parallel to the surface of contact between the rotor and the stator, and normal vibration modes, which correspond to deformations in directions normal to the surface of contact between the rotor and the stator.
Vibratory mechanical energy is taken from the tangential vibration modes to be converted into continuous movement (torque and angular speed) by the non-linear friction mechanism. These modes are therefore optimized primarily for efficient energy transfer.
In contrast, the normal vibration modes do not contribute to the final supply of energy. Their role is limited to supplying the oscillation of the rotor-stator contact bearing force needed to convert the tangential oscillatory movement of the stator into continuous tangential movement of the rotor. Consequently, the only loss of energy to which a normal oscillator is subjected is that due to its internal losses.
The tangential oscillators of the motor must be made to operate at their resonant frequency to optimize the main transfer of energy. This is known in itself.
The invention proposes to make the normal oscillators operate at their resonant frequency as well, i.e. to dimension the motor so that the normal and tangential modes have very similar resonant frequencies or even the same resonant frequency.
This minimizes dielectric losses in the active material and also minimizes losses in the electronic circuits feeding it.